The present invention relates to fuel cell systems.
Fuel cells are roughly classified into phosphoric acid type, alkaline type, molten carbonate type, solid oxide type, and solid polymer type, according to the kind of the electrolyte which they use. Among them, solid polymer fuel cells (PEFCs), which are capable of operating at low temperatures and have a high power density, are becoming commercially practical in such applications as automobile power sources and domestic cogeneration systems.
Among this type of fuel cells, direct-fuel-oxidation type fuel cells can generate electric energy by directly oxidizing fuel at the electrode, without reforming fuel that is liquid at ordinary temperature into hydrogen. Since they need no reformer and thereby permit an easy reduction in the size of power sources, they are expected to be used as the power source for various portable electronic devices. As the fuel to be fed to this type of PEFC, the use of low-molecular-weight alcohols and ethers has been examined. Most promising is methanol, which offers a high energy efficiency and a high power. Fuel cells that use methanol as the fuel are called direct methanol type fuel cells (DMFCs).
The commonly used electrolyte of PEFCs is “Nafion (trademark)”, which is a perfluorocarbon sulfonic acid polymer available from E.I. Du Pont de Nemours & Co. Inc. However, the use of such a perfluorocarbon sulfonic acid polymer as the electrolyte membrane of a DMFC causes a problem called crossover phenomenon, in which the fuel undesirably permeates through the electrolyte membrane to the cathode, significantly decreasing output voltage.
Therefore, research and development of DMFCs has been focused on the development of an electrolyte membrane that would enable a significant reduction in fuel crossover. However, the reduction in crossover currently represents a tradeoff for proton conductivity. Hence, an electrolyte membrane that realizes both reduced crossover and high proton conductivity and is suited for DMFCs has yet to be developed.
The reaction occurring at the anode of a DMFC is a reaction of fuel with water, as expressed in formula (1), whereas the reaction at the cathode is a reaction of water production from protons and oxygen, as expressed in formula (2).CH3OH+H2O→CO2+6H++6e−  (1)3/2O2+6H++6e−→3H2O  (2)
As can be seen from the above reaction formulae, it is ideal to supply the anode with a solution containing equimolar methanol and water, i.e., a 50 mol % aqueous solution of methanol. However, since the occurrence of methanol crossover is mainly due to the molecular diffusion caused by the concentration gradient of the methanol aqueous solution within the electrolyte membrane, the crossover decreases with the decrease in the concentration of the methanol aqueous solution supplied to the anode. It is therefore necessary to make the concentration of the methanol aqueous solution, supplied to the anode, lower than the stoichiometric quantity of 50 mol % as derived from formula (1), in order to maximize the output of currently available DMFCs.
However, storing a low-concentration methanol aqueous solution as fuel inside a fuel cell system leads to a decrease in volume energy efficiency, which is a fatal degradation in performance.
In view of this problem, Japanese Laid-Open Patent Publication No. Hei 5-258760 proposes a system that has a fuel tank for storing a pure fuel, a water tank, and a dilute fuel tank for storing a dilute fuel aqueous solution, such that the fuel concentration in the dilute fuel tank is controlled.
The methanol concentration in an aqueous solution can be determined by measuring, for example, the density, refractive index, or conductivity of the aqueous solution, but these measurement methods have associated problems, such as slow response speed, poor accuracy, impurity-induced errors, respectively. There are also some other methods that have been developed recently, such as measurement of transmitted infrared rays and measurement of ultrasound velocity, and instruments that measure concentration utilizing these methods are commercially available. Other proposed methods include a method utilizing the difference in heat capacity between water and fuel (Japanese Unexamined Patent Publication 2003-511833) and a method measuring the capacitance of fuel and electrolyte (Japanese Unexamined Patent Publication 2003-507859).
For the above-mentioned direct-fuel-oxidization type fuel cells, maintaining and controlling the dilute fuel concentration is indispensable to achieve stable performance. However, as described above, according to the methods of directly measuring the dilute fuel concentration, the measurement accuracy is insufficient or unstable, and concentration maintenance/control is difficult for the following two reasons.
The first reason is the influence of temperature. Generally, the values of physical properties of liquid, such as density, conductivity, and sound velocity, vary depending on temperature. It is thus necessary to accurately measure the temperature of the liquid and make a correction based on the measured value, in order to determine the concentration. However, the temperature of the fuel to be supplied to the dilute tank of a fuel cell is not necessarily the same as the temperature of the dilute fuel aqueous solution in the dilute tank. Specifically, during the time from the supply of the fuel into the dilute tank until the temperature of the dilute fuel aqueous solution becomes uniform within the dilute tank, the determination of concentration inevitably involves errors. Also, when polar solvents, such as methanol and water, are mixed, heat of mixing occurs, thereby further adding to temperature variation.
The second reason is the inclusion of impurities. For example, in a direct-fuel-oxidization type fuel cell system utilizing methanol as the fuel, carbon dioxide is generated at the anode, as shown in formula (1). Although most of the generated carbon dioxide is discharged to the outside in the form of gas, part of it dissolves in the fuel aqueous solution, because carbon dioxide dissolves in water in considerably large amounts. Also, particularly in the electrochemical reaction of methanol catalytic oxidation, it is known that formaldehyde and formic acid are generated as by-products, although in very small amounts, and they dissolve readily in the fuel aqueous solution near the electrode. The dissolution of such impurities in the fuel aqueous solution causes a change in the values of most physical properties that are utilized for measuring the concentration, such as density, conductivity, and sound velocity.
Generally speaking, with direct-fuel-oxidation type fuel cells, it is difficult for all the fuel aqueous solution supplied to their respective cells from the dilute tank to contribute to the electrode reaction, so the un-reacted fuel is often collected for reuse. Specifically, the fuel aqueous solution discharged from the electrodes is forced to return to the fuel dilute tank, so that it is used repeatedly. When the fuel is reused, the amount of the above-mentioned impurities included in the fuel aqueous solution increases as the operation of the fuel cell system continues. Therefore, the dissolution of carbon monoxide, formaldehyde, and formic acid into the fuel aqueous solution continues until it reaches saturation.
The two problems mentioned above are unavoidable in measuring liquid concentration. It is accordingly difficult to maintain and control the fuel concentration in the dilute tank of direct-fuel-oxidation type fuel cells in a stable manner, by directly measuring the fuel concentration in the dilute tank.